Laser consolidation apparatus for manufacturing precise structures

ABSTRACT

A laser consolidation apparatus for manufacturing precise three dimensional structures disclosed. The apparatus comprises means for feeding a stream of material to an area on a surface to be built up, at an angle normal to the surface and means to direct a converging laser beam on the surface at an angle of  25  to  30 ° to the normal to the surface from a plurality of directions surrounding the material feed. Discrete mirror segments direct the laser beam on to movable plane mirrors producing reflected beams which combine to illuminate an area on the work surface.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 09/699,483 filed Oct. 31, 2000 entitled “Laser ConsolidationMethodology and Apparatus for Manufacturing Precise Structures” which isa continuation-in-part of application Ser. No. 09/408,853 filed Sep. 30,1999 and entitled “Process for Manufacturing or Repairing Turbine Engineor Compressor Components”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] This invention relates to an apparatus for building precise 3Dcomponents and structures by a material addition process called laserconsolidation, more particularly an arrangement for the verticaldelivery of metallic powder, or wire into a precisely formed melt poolcreated in a substrate by laser beams having a specific angularorientation relative to the substrate.

[0005] 2. Background Information

[0006] Rapid Prototyping (RP) is a related technique based on layeredmanufacturing where a part is built as a series of horizontal layers,each one being formed individually and bonded to the preceding layer.Various processes have been used differing in the way each layer isformed and the raw materials used but the underlying methodology isessentially the same in each case.

[0007] Stereolithography (SLA) and Selected Laser Sintering (SLS) arethe two most common rapid prototyping processes. In both cases, a threedimensional CAD model of a part is generated and sliced into horizontallayers. The sliced files are used for tool path generation to make asolid part layer by layer. The thickness of each slice is controlled andis determined by the degree of accuracy required and the capability ofthe system, viz-a-viz the maximum thickness that can be cured orsintered by the specific process.

[0008] The SLA process uses a photosensitive monomer, which is curedlayer by layer using an ultraviolet laser resulting in a cured polymerpart. In the SLS process a carbon dioxide laser of appropriate power isused to scan across the surface of a bed of a powdered thermoplasticmaterial, sintering the powder into the shape of the requiredcross-section. A major limitation of the SLS process is itsinflexibility in the selection of metals that can be used. To generatemetallic parts, thermoplastic coated metal powders are used to create a“green shape” of the component. The thermoplastic plastic is removed ina “burn-off” step and replaced by infiltrating a lower melting pointmetal.

[0009] In order to produce dense three dimensional metal/alloy parts,Los Alamos National Laboratory in the U.S. developed a process called“Directed Light Fabrication of Complex Metal Parts” (1994 ICALEOconference). In this process a coaxial powder delivery nozzle is usedwith a normal laser incident angle. The focussed laser beam enters achamber along the vertical axis of the nozzle that also delivers metalpowder to the focal zone. The deposition is done on a base plate, whichis removed after the part is built. The powders used for part build-upare 316 stainless steel, pure tungsten, nickel aluminide and molybdenumdisilicide.

[0010] In a paper presented at a “Rapid Prototyping and Manufacturing“96” conference (SME, Michigan, Apr. 23-25, 1996) Dave Keicher of SandiaNational Laboratories dealt with “Laser Engineered Net Shaping (LENS)for Additive Component Processing”. This process uses a Nd:YAG laser anda special nozzle arrangement for powder delivery. Four streams of powderare fed into a melt pool which is created and sustained by a centrallaser beam. It is pointed out that this arrangement avoids the situationin off-axis single side feed powder delivery system where there is astrong directional dependence. The symmetrical (quasi coaxial)arrangement permits uniform cladding independent of direction.

[0011] A rapid prototyping technique has also been used by Prof. W.Steen (1994 ICALEO conference paper). A machining pass is added aftereach build-up pass, and a high power carbon dioxide laser (>2 kw) isused. Optics for the beam delivery system are incorporated on anautomatic tool changing system. The process requires that after eachlaser build-up pass, the metal layer is machined back to requireddimensions, necessary because of a lack of control on the laserbuild-up. It was also found that a change in cladding direction has asignificant influence on the shape and quality of the build-up. Goodquality clad with a regular shaped bead was obtained parallel to theflow direction but as the angle to the flow direction increased thequality deteriorated until clad perpendicular to the flow was of poorquality. Machining is used to remove the imperfections in shape and sizeof each built up layer arising from the change in the clad direction. Asside nozzle powder delivery builds unevenly in various directions in thexy-plane, the additional required step of machining after eachdeposition pass makes the process cumbersome and expensive. As thecontrol on the build-up process is poor, most of the material is removedto maintain the geometry creating unnecessary waste of expensivematerial.

[0012] It is evident from the above that in building up metal partsusing a carbon dioxide or Nd:YAG laser and metallic powder, singlenozzle side delivery always involves a directional dependence, and iseither abandoned in favour of coaxial powder delivery or machining isemployed after every pass to maintain dimensions. The trend is to use acoaxial powder delivery to obtain equal layer buildup in all directions.In addition it is apparent that the incident laser beam is always normalto the surface of the base plate.

[0013] Several nozzle designs for coaxial powder feeding during lasercladding have been disclosed, for example: U.S. Pat. No. 4,724,299(Hammeke, Feb. 9, 1988); U.S. Pat. No. 5,418,350 (Freneaux, May 23,1995); U.S. Pat. No. 5,477,026 (Buongiorno, Dec. 19, 1995) and U.S. Pat.No. 5,111,021 (Jolys, May 5, 1992).

[0014] U.S. Pat. No. 5,731,046 to Mistry (Mar. 24, 1998) discloses atechnique for fabricating diamond and diamond-like coatings on asubstrate. Mistry also discloses that complex shapes can be fabricatedas coating structures on the surface of the substrate. Mistry disclosesusing a plurality of lasers each having different and specific temporaland spectral characteristics to perform the following functions: onelaser to ablate the constituent element, a second to initiate chemicalreaction, and a third to provide overall thermal balance. Mistrydiscloses that shaped coatings can be made on the surface of thesubstrate by the relative movement of the laser system and thesubstrate. Ministry does not teach the importance of the critical angleof the lasers relative to the powder feed nozzle, the symmetricalarrangement of the laser beams relative to the material feed system northe control over and the shape of the melt pool required to make precisestructures and components with smooth walls.

[0015] The inventors' U.S. Pat. No. 5,855,149 (Canadian application2,242,082 published Dec. 30, 1999) teaches a method of producing asharpened edge on a cutting die by having a laser beam or beams impingeon a base surface at an angle to the normal of between 5° and 45° tofuse successive thin layers forming a metal ridge to the cutting edge.The inventors' Canadian application No. 2,215,940 published Mar. 23,1998 discloses an apparatus and method for material disposition on asurface using a laser beam or beams impinging on the surface at an angleto the normal of between 5° and 45°.

[0016] Generally laser based material addition processes rely onfocussing a laser beam to create a small molten zone in a suitablestarting material (substrate). New material, usually in powder form, isadded and melted to increase the volume of the molten zone. When thelaser is shut off, or moved to a new location, the molten materialrapidly cools and solidifies. When the process is sustained by movingthe laser and material addition system across the substrate, at acontrolled speed, it is possible to make a uniform ridge. The ridge cantake on geometric forms when the laser and powder feed systems are movedacross the substrate by following a predetermined path as described by acomputer numerically controlled system. By repeating the operation usingthe original ridge as a new substrate, eventually after subsequentlayers are added, a walled structure is formed.

[0017] All of the processes reported, can be described as near netshape. For example, Sandia National Laboratories, using their LaserEngineered Net Shape (LENS™) process, can produce parts with complexshapes having surface finishes that resembles a fine sand casting andhaving dimensional tolerances at best of +/−100 microns. To obtainbetter dimensional control and surface finishes requires secondaryoperations.

[0018] The arrangements commonly used in the prior art as illustrated inFIG. 1 of the drawings have a central laser source 20 with powder 21entering symmetrically from the sides around the circumference. In thisway the relationship to the pool remains the same regardless of the wallpath. The arrangement is symmetrical but powder entering from the sidescauses thermal and viscosity gradients 22, leading to incomplete meltingwhere the wall surfaces are forming. Line 23 indicates the boundary ofthe molten zone. Unmelted or partially melted particles of powder 24tend to stick on the surface as the wall is cooling. Attempting tocorrect the situation by adding more energy is not successful becausethe surface where the energy enters starts to evaporate causing a plasmato form which absorbs the incoming laser energy. The mass of “soupy”unmelted material in the vicinity of sides of the wall tends to slumpoutside the dimensions of the pool. Subsequent passes, or layers,applied in this slumped condition result in a wall where each layer hasa convex curved surface 25. These curves at the surfaces of the layersproduce variations in the wall thickness 26. The resultant wall has theappearance similar to 27 shown in FIG. 1a.

[0019] The practice of making precise structures in the prior art is toform a rough shape then use a material removal operation such asmachining to create the final shape and surface finish. The presentapplication describes a methodology and apparatus for making precisestructures, for example, in the form of shells, in one operation.

[0020] When a focused laser is used to rapidly melt a zone in asubstrate, and the zone is cooled quickly, the surface of the solidifiedzone is smooth. When the melting takes place in a non-oxidizing, dustand vibration free environment, and the molten zone is maintained closeto the flow temperature of the substrate material, sub-micron finishescan be obtained on the solidified surface. If the melting process iscontrolled it is possible to get high quality surface finishes.

[0021] When material is added and melted into the melt pool, to increaseits volume, it is more difficult to maintain a smooth finish. Theproblems with existing state of the art near net shape processes thatfeed powder into the pool from the sides stem from the thermal, andhence viscosity gradients, created in the pool and from powder particlessticking to the side walls as the pool solidifies.

SUMMARY OF THE INVENTION

[0022] In the Invention

[0023] Laser energy enters the molten pool at an angle of about 30degrees to the vertical and symmetrically around the pool in the form ofan annulus. Powder is injected vertically at the top dead centre of themelt pool through a fine nozzle.

[0024] The Advantages Are:

[0025] In forming the pool of molten material, energy enterssymmetrically around the pool allowing the temperature to rise uniformlyand rapidly avoiding local evaporation or the creation of seriousthermal gradients within the pool.

[0026] Surface tension is maintained uniformly around the pool and henceresults in a pool with a surface that is close to hemispherical inshape.

[0027] The temperature of the pool in the regions where the walls willform is uniform from side to side and is controlled above the melttemperature so that all the powder is completely melted. Thus the wallsformed on cooling have a precise width and the surfaces are smooth.There are no visible or metallurgical discontinuities to show that thestructure has been formed in a series of passes.

[0028] Directing the powder into the pool at the top ensures a highcapture rate of powder and any stray particles are directed through theincoming beam and away from the solidifying wall surfaces.

[0029] The symmetry of the total system permits the substrate to bemoved in any direction relative to the laser powder feed arrangementwithout changing the thermal balance within the melt pool.

[0030] The apparatus of this invention meets the criteria for makingprecise walls. However, in practice there may be a need to make minoradjustments in wall thickness. A fixed focus rigid 360 degree focussingmirror precludes any adjustment.

[0031] Variations in the apparatus are disclosed which permit fineadjustments. Specifically, the invention relates to apparatus fordeposition and machining of an article comprising: a laser, focussingmeans producing a convergent beam from the laser, and reflecting meansdefining a path resulting in the focal point of the beam beingpositioned close to a work surface. The focussing means is adapted formovement whereby the focal point can be moved above and below the worksurface for improved precision.

[0032] In another aspect the invention relates to apparatus fordeposition and machining of an article comprising: a laser, focussingmeans producing a convergent beam from the laser and means adapted formovement of the focussing means to vary the position of the focal pointof the beam. A beam splitter and reflective means define paths for thesplit beams to impact on a work surface. The reflective means includes aplane reflecting surface adapted for movement in a direction to alterthe length of the paths. When the beams overlap on the work surface theyproduce an area of enhanced energy useful for cutting, welding ormachining.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The foregoing, and additional objects, features, and advantagesof the present invention will become apparent to those of skill in theart from the following detailed description of preferred embodimentsthereof, taken with the accompanying drawings, in which:

[0034]FIG. 1 Is a schematic diagram showing sources of dimensionalinaccuracy and surface roughness in prior art equipment.

[0035]FIG. 2 Contains schematic diagrams illustrating surface tensionforces.

[0036]FIG. 3 Shows a cross-sectional view of 360° focussed laser beamconsolidator.

[0037]FIG. 4 Shows the direction and arrangement of laser beams andpowder feed.

[0038]FIG. 5 Shows a cross-sectional view of the apparatus using apyramidal mirror,

[0039]FIG. 6 Shows a cross-sectional view of apparatus using lenses,

[0040]FIG. 7 Shows a variation of the apparatus shown in FIG. 5 usingplane rather than spherical reflectors,

[0041]FIG. 8 Shows the beam paths in the apparatus of FIG. 7 when thefocussing lens is moved, and

[0042]FIG. 9 Shows the beam paths in the apparatus of FIG. 7 when theplane reflecting mirrors are moved.

DESCRIPTION OF THE INVENTION

[0043] In building components using the laser consolidation process, theshape and position of the liquid to vapour and the liquid to solidsurface tension interfaces are critical.

[0044] Using the consolidation process to create precise structures withsmooth sides, requires the volume of molten material to be as close tospherical as possible where the diameter of the sphere is equal to thethickness of the wall under construction. It follows that the processdepends on being able to control the diameter and location of thesphere.

[0045] It is well known that when a liquid body is free to do so, itminimizes its energy content by assuming a spherical shape. This is theshape that has the least surface area to volume. In practice this isdifficult to attain. One example, is when a liquid freezes during a freefall, such as in forming lead shot. It is also possible using thecontrolled conditions of the consolidation process to create essentiallya spherically shaped volume of molten material.

[0046] From a theoretical point of view, to attain vertical and smoothside wall construction, requires the shape of the molten surface exposedto the atmosphere to be essentially hemispherical. It is equallyimportant that the portion of the molten zone cradled within thesubstrate is also essentially hemispherical, so the total molten volumeis spherical. This spherical condition results in a smooth verticaltransition as each pass of material is added. In the spherical conditionthe components of surface tension, liquid to solid (yLs), liquid tovapour (yLv) and solid to vapour (ySv) are balanced. Various models ofmelt pool configurations are shown in FIG. 2, in which this balance isnot maintained and where material slumping is experienced. Slumping iscaused by the mass of added material overcoming the surface tensionforce's effort to form a sphere.

[0047] To aid in achieving the total spherical shape it is necessary tocreate within the wall, or substrate, a cradle which is essentiallyhemispherical to support the liquid. It has been found experimentallythat the molten material has to be cradled in such a way that the liquidto solid surface tension interface (Ls) blends smoothly and verticallyinto the liquid to vapour surface tension interface (Lv). Since thereare no angular forces in this condition the surface tension force systemcan be expressed as yLv=yLs+ySv. Because the consolidation process formsa cradle with a sharp edge at the wall surface, ySv is negligible, thusyLs=yLv which is what one would expect in a perfect sphere. If thesmooth vertical blend is not achieved some form of aberration will occurin the forming side wall.

[0048] The applicants have found that to create a suitable supportcradle the entry angle of the laser beam has to be between 25-30 degreesto the vertical. It may be possible to use smaller angles but practicallimitations imposed by the powder feed system prevented exploration ofthis logical possibility. Increasing the angle produces shallowercradles and an imbalance in the surface tension force system resultingin discontinuities in the wall surfaces.

[0049]FIG. 2 illustrates surface tensions components for varioussystems. In FIGS. 2a, 2 b, 2 c, 2 d the liquid-to-vapour interface (Lv)is represented by arrow 50, the liquid-to-solid interface (Ls) isrepresented by arrow 51 and the solid-to-vapour interface (Sv) isrepresented by arrow 52. In all of the conditions shown in FIGS. 2a, 2b, 2 c (representing prior art) the surface tension components haveangular relationships to each other and can be expressed as yLv+yLs cosθ₁+ySv cos θ₂=θ, where θ₁, θ₂ represent the angular relationshipsbetween the surface tension components.

[0050] In FIG. 2a (prior art) the shallow cradle 53 limits the build upcapacity because of the onset of slumping, or overflowing. In FIG. 2b(prior art) the laser energy distribution has created shallow areas 54near the sidewalls and is unable to support the build up of material. InFIG. 2c (prior art) the deeper cradle 56 improves the build capacity,but the surface tension forces in attempting to attain a spherical formcause the material to bulge 57 resulting in poor dimensional control andundulating surfaces. In FIG. 2d (representative of the presentinvention), the hemispherical cradle allows material to build up suchthat the surface tension components 50, 51, 52 at the point of maximumbuild up are vertically aligned and thus surface tension is able to forma spherical pool.

[0051]FIG. 3 shows a 360 degree focussed laser beam consolidator 30. Alow energy density expanded laser beam 31 enters at the top of theconsolidator 30. The laser beam is reflected by a conical mirror 32forming a divergent hollow conical beam 33 which impinges on a 360degree spherical mirror 34. The spherical mirror reflects the hollowconical beam in the form of a convergent hollow cone 35 and focuses theenergy in an annular pattern 36 on the substrate surface 37. The angle δthat the converging beam makes with the vertical axis of the system is30 degrees. The diameter of the melt pool made by the annular ring ofenergy determines the wall thickness of the structure to be built. Apowder feed tube 38 passes through the centre of the low energy densityincoming laser beam and through the centre of the conical mirror andterminates in a precision powder feed nozzle 39 positioned directlyabove the melt pool. Powder is propelled through the powder feed tubeand injected from the nozzle into the centre of the melt pool.Additionally the consolidation system features a gas purge system 40 tokeep the mirrors clean and as the gas exists from the nozzle of theprotective cone 41 provides a cover gas 42 over the melt pool thatinhibits oxidation.

[0052]FIGS. 4a and 4 b illustrate the arrangement for the delivery ofenergy and powder. Laser energy 70 enters the substrate material 71symmetrically in an annular pattern from several directions up to a full360 degrees at an angle of 30 degrees to the normal of the substratesurface, or vertical axis. The cumulative effect of the energy in thebeams creates within the substrate a hemispherical pool of moltenmaterial 72, the viscosity of the molten material is relatively constantthroughout the pool. A controlled stream of powdered material 73 isinjected into the top of the molten pool 74. The volumetric rate ofmaterial entering the pool is critical to ensure that the mass of theincreased height is balanced against the ability of the surface tensionforce to maintain the spherical shape 75 of the pool. Subsequent passesare required to build-up a wall of material. The height of each pass 76is small and consistent with the balancing of surface tension againstgravity. Because the region close to the edge 77 is kept in a moltenstate, the surface tension in the material is able to maintain theverticality of wall between passes. Because of the small mass of moltenmaterial, relative to that of the substrate, the material solidifiesquickly leaving a smooth surface 78 and a fine grained metallurgicalstructure in the material.

[0053] By replacing the conical mirror shown in FIG. 3 with amultifaceted pyramidal mirror, a number of beamlets equal to the numberof facets will be reflected on to the concave spherical mirror. Thebeamlets are then focussed by the spherical mirror to form an annulus ofenergy in the melt pool that is similar to the 360 degree arrangementshown in FIG. 3. The multi-beamlet arrangement has the advantage ofproviding space for the powder feeder or sensors to enter between thebeamlets, and thereby simplify the construction of the consolidationsystem.

[0054] In the multi-beamlet configuration individual mirrors for eachbeamlet can replace the single spherical mirror FIG. 5. The incomingbeam 80 is split into four beamlets by the four sided pyramidal mirror82. The four beamlets with D-shaped cross-sections are reflected towardsthe individual mirrors 81 that have a concave spherical surface. Thebeamlets in turn are reflected and focussed onto the substrate 83. Theindividual mirrors can be moved horizontally so that the axis of thefocussed beamlet is shifted laterally in, or out. This lateraladjustment is used to change the melt pool diameter and hence thethickness of the part being produced. The amount of movement is small,in the order of 13 microns (0.0005″) and the focussing angle is kept at30 degrees. The arrangement also shows the powder feed tube 84 enteringthe system between the beamlets and connecting to the injection nozzlebelow the pyramidal mirror.

[0055] It is further possible to reduce the number of facets on thesplitter mirror to two, that is, a wedge rather than a cone. Thistwo-beamlet arrangement produces less precise parts, as the energydistribution in the D-shaped beamlets is not completely uniform duringdirectional changes.

[0056] The two-beamlet arrangement is capable of producing a nominalwall thickness controlled to +/−25 microns and surface finishes betterthan 2 microns Ra.

[0057] Standard optics can be used to perform in a similar manner to themulti-beamlet approach, such as that shown in FIG. 6. This apparatusemploys lenses 91 positioned to focus supplied laser beams at thedesired focussing angle δof 30 degrees. The powder feed system isdirectly vertical 92. The optics and the powder feed system areaccurately position in a rigid body 93 that can also support aprotective feed cone 94 and an inert gas cover purge and system 95 and96. The number of beams that can be physically accommodated is limitedin this design. Energy 97 may be supplied to the focussing lensesthrough fibre optic delivery systems commonly used with Nd:YAG lasers. Astructure 98 is shown being built on an original substrate 99.

[0058]FIG. 7 illustrates a modification of the apparatus shown in FIG. 5in which the spherically mirrored sections 81 are replaced by movableplane mirrors 101. A movable focussing lens 102 is provided in the pathof the laser beam prior to the beam impacting on the flat facetedsurfaces of the reflector 82, which as shown is pyramidal-shaped.

[0059] Movement of the focussing lens 102 results in movement of thefocal point of the beam 103 above and below the work surface, as isshown in the ray diagram of FIG. 8. A dx displacement of the lens 102leads to a similar displacement of the focal point. Movement of thereflecting plane mirror 101 results in sideways displacement of the beamwith respect to the work surface as shown at 104 in the ray diagram ofFIG. 9. A displacement d in the mirror 101 leads to a slightly largerlateral displacement d′ of the focal point, where d′=2d cos θ. Thisarrangement provides improved control with a structure less complex thanthat shown in FIG. 5. Although the movement of mirrors 101 is shown asbeing perpendicular to their planar surface, which facilitates controlthereof, it will be appreciated that the movement could be lateral orradial with respect to the central axis of the apparatus.

[0060] A further advantage of the apparatus of FIG. 7 is that it ispossible to have the separate beams overlap originating at the beamsplitter on the work surface thereby producing increased intensity whichis useful for cutting, welding and machining.

[0061] It will be clear to persons skilled in the art, that othernumbers of beams could be used and would fall within the scope of thisinvention. It will also be clear to those skilled in the art thatmaterials other than powdered metal could be used and that other formsof material such as wire could be used in the material feed withoutdeviating from the scope of the present invention.

1. Apparatus for consolidation and machining of an article comprising: alaser; focusing means producing a convergent beam from said laser, meansadapted for movement of the focussing means to vary the position of thefocal point of the beam; means splitting said beam including reflectivemeans defining paths for the split beams to impact on a work surface,the reflective means including a plane reflecting surface adapted formovement in a direction to alter the length of the paths; whereby theposition of the split beams is precisely controllable and overlappableon the work surface to produce an area of enhanced energy.
 2. Apparatusfor deposition of layers of material on a work surface, comprising: amaterial feed extending normally to the work surface; a laser; focusingmeans producing a convergent beam from said laser; reflective meansdefining a path for said beam to impact on a work surface; thereflective means including a plane reflecting surface adapted formovement in a direction to alter the length of the path to vary theposition of impact of the beam on the work surface, whereby the materialis melted and maintained in molten spherical configuration by surfacetension.
 3. Apparatus for deposition of layers of material on a worksurface, comprising: a material feed extending normally to the worksurface; a laser; focusing means producing a convergent beam from saidlaser; reflecting means defining a path resulting in the focal point ofsaid beam being positioned close to a work surface; means adapted formovement of the focusing means whereby the focal point is moved aboveand below said work surface; whereby the material is melted andmaintained in molten spherical configuration by surface tension. 4.Apparatus for deposition of layers of material on a work surface,comprising: a material feed extending normally to the work surface; alaser; focusing means producing a convergent beam from said laser, meansadapted for movement of the focussing means to vary the position of thefocal point of the beam; means splitting said beam including reflectivemeans defining paths for the split beams to impact on a work surface,the reflective means including a plane reflecting surface adapted formovement in a direction to alter the length of the paths; whereby thebeams overlap on the work surface producing an area of enhanced energyand the material is melted and maintained in molten sphericalconfiguration by surface tension.
 5. Apparatus for manufacturing precisestructures, comprising: a powder feed substantially normal to a worksurface; a laser; focusing means producing a convergent beam from saidlaser; reflective means defining a path for said beam to impact on awork surface, the reflective means including a plane reflecting surfaceadapted for movement in a direction to alter the length of the path tovary the position of impact of the beam on the work surface, said beambeing at an angle of 25° to 30° to the normal, producing an annularpattern on the work surface, substantially surrounding the powder feed.6. Apparatus for manufacturing precise structures, comprising: a powderfeed substantially normal to a work surface; a laser; focusing meansproducing a convergent beam from said laser, reflecting means defining apath resulting in the focal point of said beam being positioned close toa work surface; means adapted for movement of the focussing meanswhereby the focal point is moved above and below said work surface; saidbeam being at an angle of 25° to 30° to the normal, producing an annularpattern on the work surface, substantially surrounding the powder feed.7. Apparatus for manufacturing precise structures, comprising: a powderfeed substantially normal to a work surface; a laser; focusing meansproducing a convergent beam from said laser, means adapted for movementof the focussing means to vary the position of the focal point of thebeam, means splitting said beam including reflective means definingpaths for the split beams to impact on a work surface, the reflectivemeans including a plane reflecting surface adapted for movement in adirection to alter the length of the paths; said beams being at an angleof 25° to 30° to the normal, producing an annular pattern on the worksurface, substantially surrounding the powder feed, whereby the beamsoverlap on the work surface producing an area of enhanced energy.